So far there have been known some RF based EAS systems such as those disclosed in U.S. Pat. No. 4,509,039. Such apparatus radiate a radio frequency field at a specified frequency in the surveillance zone through which the article may pass, and produces an alarm upon the detection of current induced by a resonant circuit within the marker, which is secured to the article.
A resonant circuit in the marker is typically composed of a flat or planar coil in combination with a capacitor. In order that the system can detect the marker, it is necessary that a radio frequency field radiated from the system pass through the coil. However, if the marker is positioned adjacent to an article having a metallized or otherwise conductive surface, such as covered with aluminum or other metal films, metal foils or metal plates, the conductive surface will shield the coil from the radio frequency field and thereby cause the distance from the system at which the marker can be detected to become so short as to be unacceptable.
A method which previously has been employed for preventing the marker from overlapping an article such as CD and thereby being shielded from radio frequency fields, uses a hard case 11 as shown in FIG. 3. The hard case 11 consists of a portion A for receiving the CD and a portion B neighboring this. The portion B is provided with a tag pocket 12 into which a marker containing a planar coil 1 is placed.
However, many problems exist with the method mentioned above, such as the high cost due to the special requirement of a hard case, the need for a space within the hard case, and others.
Accordingly, the object of the present device is to solve the problems of the previous methods as mentioned above and to provide a marker for radio frequency EAS systems which is low cost and requires no special space.